1. Field of the Invention:
The present invention relates to color-developing plated metal for springs and the method of using the same, and more specifically, to a color-developing plated metal for springs capable of being suitably distinguished in size, material and the like and the method of using the same.
2. Description of the Prior Art
A product formed of spring steel (that is, a spring) such as a coil spring or a sheet spring is used in various applications such as mechanical parts, official materials and daily necessities. The spring steel as a material for the above spring includes a spring steel wire and a spring steel sheet. As the spring steel wire, there are known a hard drawn steel wire, a piano wire and a spring stainless steel wire specified in Japanese Industrial Standard (hereinafter referred to as JIS).
These steel wires resemble each other in surface color tone, and particularly, a hard drawn steel wire cannot be distinguished from a piano wire by only the color tone. Also, in the case of a stainless steel wire, it generally is more lustrous as compared to a hard drawn steel wire and a piano wire; however, when being finished by oil drawing (wet drawing), it cannot be distinguished by color tone. Accordingly, of these steel wires after spring-forming, those resembling in size to each other have often created a problem of their mixing in size or material, and consequently, the defective spring product being liable to be erroneously assembled in a mechanical structure.
On the other hand, a steel cord for reinforcing a radial tire of an automobile is formed as follows: namely, five elementary wires each having a diameter of, for example, 0.25 mm are stranded, and the stranded wire is knitted into a belt-shape and is disposed around the periphery of the tire. Thus, the steel cord aims at reinforcement of the radial tire as a rubber-metal cord composite material. The above elementary wire is manufactured by the steps of: applying a Cu-plating as a lower layer and a Zn-plating as an upper layer on the surface of a raw wire having a diameter of 1.3mm at a plating thickness ratio of Cu:Zn=7.3; heating the plated wire at approximately 400.degree. C. for a few minutes to tens of minutes for alloying the plating layers into a Cu-30% Zn alloy; and forcibly drawing it at a reduction ratio of 96.3% to a diameter of 0.25 mm. During such processes, after heating, the color of the plating surface is changed from white to gold, which exhibits a very beautiful color tone.
In the manufacture of a steel wire mentioned above, the fact that the surface color tone of the cord is changed to gold is worthless, and the object is to improve the drawability and adhesiveness between rubber and metal by alloying the plating layer into a Cu-30% Zn alloy. Accordingly, the positive function of the coloring generated by plating the material with two different metals and applying thermal diffusion thereto was not recognized.
In addition, the steel coated with only a plating layer of Cu-30% Zn alloy has no problem in terms of corrosion resistance when it is embedded in rubber, for example, as in the case of the steel cord and thus shielded from the outside air. However, in the case of using the above steel as a formed product without shielding it from the outside air, it is insufficient for corrosion resistance and results in problems.
To prevent the above products formed of spring steel of different size and material form mixing and also to improve their beautiful appearance, there have been used the following coatings on the spring steel wire: various resin film coatings; baked coatings of paint; ion plating by PVD or CVD; and TiN coating.
However, in spring-forming, the spring steel wire is subjected to severe abrasion close to galling in passing through the forming tool, and is also subjected to a heat treatment (low temperature annealing) at 250.degree. C.-400.degree. C. for 2-10 min. after spring-forming for improving the spring characteristics. Consequently, the spring steel wire coated with a resin film or baking paint is liable to be damaged on the surface thereof during the spring-forming thereby causing peeling of the film, and also the film is softened during the low temperature annealing thus causing depressions in the film and mutual adhesion of the springs. The spring steel wire plated by ion plating does not result in the above problems; but has a disadvantage of increasing the cost. Therefore, the known techniques do not function in an adequate manner.